This... is fortuitous I suppose, but my cofounder is actually the one who identified the vertical AND brought the first customer/partner. I was the execution guy who also happened to be a domain expert in a relevant field.

However, to get to this point: this was startup #3 for us together (and monetizeable side project #15~20, by this point in life). Our first startup fizzled out over time as the vertical became efficient (right domain though); our second was poorly executed and run (totally my fault)... but we grew together as a team. And on collective try #3 we learned through our fuckups to not do them again.